The present invention relates to a source driver circuit of active matrix organic light emitting diode displays (AM-OLED), more specifically, to a digital to analog (D/A) converter for a current-driven type source driver circuit.
With rapid advancement of the fabrication technology of thin film transistors, the OLEDs in new generation are widely applied in various portable electronic products for providing more fine images due to the fact it has advantages of high emitting efficiency, fast responding rate, power saving, non-restriction of visible angle, smaller size, less weight, high brightness and all-color images.
Please refer to FIG. 1, the circuit structure 10 of each unit pixel in current-driven type OLED device is shown. When the signal on the scan line is at low potential level, the PMOS transistors P1, P2 and P3 are sequential switched on. Because the transistor P3 connected to the voltage sourcexc2x7Vdd can conduct with the current source 15 on the data line via the transistor P2, the drain current of transistor P3 will be equal to the current Idata of the current source 15. For providing the required current Idata of the transistor P3, the capacitor 20 connected to the gate of the transistor P3 can be charged to adjust the gate potential of the transistor P3 and to provide a desired Vgs between the gate and the source thereof. Then, when the signal on the scan line is at high potential level, the PMOS transistors P1 and P2 are switched off, and the NMOS transistor N1 is switched on. At this time, the charged capacitor 20 can maintain the potential difference between the gate and the source of transistor P3 at a specific value Vgs, so as to provide a drain current of Idata and to drive the light emitting diode 25 to emit light via the conducting NMOS transistor N1.
In general, for providing more harmonizing and uniform colored images, the data current Idata on the data line is adjusted to have multiple gray levels. Please refer to FIG. 2, the current source 30 for producing the current with 16 gray levels designed by J. Kanicki is illustrated. The current source 30 comprises four transistors M1, M2, M3 and M4 with the W/L (channel width to channel length) ratio of 1:2:4:8. And the sources of these transistors are all connected to a common voltage source VD0, as to the drains thereof are connected to another transistor DR, so as to have these drains at the potential level of the reference voltage VDR0 when the transistor DR is switched on. The gates of transistors M1xcx9cM4 are connected separately to control terminals of VD1, VD2, VD3 and VD4, and switched on or off depending on the voltages of these control terminals. It is noted that because these transistors (M1xcx9cM4) have different W/L ratios individually, the drain currents thereof also have the ratio of 1:2:4:8. Thus, by turning on some transistors, the sum of currents Idata can has the desired gray level. For instance, when only one transistor M1 is turned on the data current Idata is equal to I; and when the M1 and M2 both are turned on, the data current Idata is equal to 3I.
However, it is noted that the aforementioned current source has many disadvantages. First, for manufacturing the four transistors with the W/L ratio of 1:2:4:8, it is very important to control precisely the layout dimension of each transistor. Under this condition, any inaccuracy occurring in the process will cause the W/L ratio deviate and have the severe gray level shift. Especially, when more current gray levels are desired, it is necessary to manufacture the transistors with a large W/L ratio, that will cause the layout and design of transistors more difficult. For example, when 32 current gray levels are desired, it is required to fabricate two transistors with 16 times dimension difference, for producing the W/L ratio of 1:2:4:8:16. Second, because so far most LEDs are fabricated on the low temperature poly-silicon substrates, it is difficult to manufacture the thin film transistors that meet the required specifications and parameter due to the affections of the substrate properties.
Under such conditions, the distribution of current gray levels maybe have uneven shift due to the errors of the threshold voltage and mobility of the transistors M1xcx9cM4. Especially, as shown in FIG. 2, only four transistors are applied to produce 16 current gray levels, so any one transistor with shift parameters will affect the distribution of the current gray level and the uniform degree of the LED display. Further, as shown in FIG. 1, all pixel devices on the same data line have the currents supplied by one identical set of transistors (M1xcx9cM4), therefore these four transistors would degrade very fast so as to cause the threshold voltage and mobility of each transistor change severely and to decrease the uniform degree of the LED displays, and even to alter the gray level of images.
For overcoming the above issues effectively, in some process of fabricating LED displays, the approach of thermometer code current cell decoder is introduced. Please refer to FIG. 3, a set of thermometer code current cell decoder 35 with six bits is illustrated. This thermometer code current cell decoder 35 comprises a column decoder 351, a row decoder 352, and a plurality of current cells 36 arranged in an 8xc3x978 square array form. Each current cell 36 has a local current source 361 and a local decoder 362.
For the aforementioned current cell decoder 35 of six bits, the less signal bit (LSB) D1, D2, and D3 are direct input to the column decoder 351. Because each signal bit possess the binary 0 or 1, eight sets of different column selection signals (C) are produced after the decoding procedures by the column decoder 351. As to the more signal bit (MSB) D4, D, and D6 are input to the row decoder 352, so as to produce eight sets of row selection signals (Ri) after decoding procedures. When these column selection signals Cj or row selection signals Ri are input to the current cell 36, through the logical operating circuits of the local decoder 362, the current source 361 can be decided to turned on or off for outputting current Io. For example, the local decoder 362 can be applied to perform the logical computing of ((Ri and Cj) or Ri+1), for deciding to output the current Io or not.
It is noted that, because each current cell 36 for producing output current Io has the identical current source, the multiple current gray levels can be produced according to the number of conducted current cells. For instance, when fifteen current cells are turned on, the summed data current Idata is equal to 15 Io; or when twenty-six current cells are turned on, the data current Idata is equal to 26 Io. Thus, the aforementioned issues about uniform degree of displaying images due to the variations of threshold voltage and mobility of the TFTs can be improved. Besides, because it is not applying the current source constitute of the four transistors (M1xcx9cM4) to supply the pixels on one identical data line, the operating times of each transistor are reduced considerably, so as to improve the degradation of transistors in the prior art.
However, it is necessary to reserve a considerable large area on the substrate for fabricating a large number of decoders and current cells when the approach of thermometer code current cell decoder is introduced. Especially with the increasing gray-scales of the LED displays (6 bits, 8 bits.), the whole area occupied by the current driven circuit also increase fast proportional to the square of bits. Therefore, the approach still cannot be applied to or satisfy with the displays with higher gray-scales.
Additional, such thermometer code current cell array are operated to conduct the specific number of the local current sources one by one according their arrangement and sequence for producing the desired current gray level. As shown in FIG. 3, when the current of 26 Io is desired, the first twenty-six local current sources are turned on from left to right and from top to bottom in the cells array. Therefore, for the upper half of the current cells in the array, their operating times apparently exceed that of the lower half current cells. For example, the operating times of the first one current cell located at the upper and the left side in the array are about 64 times of that of the last one at the bottom and right side in the array. Thus, after long time operating, the quality and uniform degree of the display still are affected due to the degradation each current cell suffering is very different.
The prime objective of the present invention is to provide a digital to analog current source for supplying the pixel devices of the current driven OLED display a stable data current.
Another objective of the present invention is to provide a display panel with different local current sources and thermometer code current cell decoder unit cell array.
A D/A converter applied in current driven type source driver circuits for producing the data current with different gray levels is disclosed. The D/A converter comprise the following components. A column decoder is applied to transform input digital control signals (Dx) to column selecting signals (Gj). A row decoder is applied to transform the digital control signals (Dx) to row selecting signals (Ri). A unit cell array has current cells arranged in array form. Each current cell has a local current source and a local decoder, the local current source is chosen from a plurality of current sources having different current intensity, and the local decoder can decode the column selecting signals and the row selecting signals for deciding to switch on or off the local current source thereof. The output currents of all the current cells switched on are summed up to produce the data current with a selected gray level.
It is noted that the different current intensity for the local current source comprise 2NI (I, 2I, 4I. . . ), wherein the N is positive integer, and the I is a lowest output current. The current source also can be adjusted to provide I, 2I and 3I depending on the requirements of design and layout. Besides, when the digital control signals (Dx) possess X bits, the number of the current cells is xe2x89xa6X2. And when the digital control signals (Dx) possess X (Xxe2x89xa63) bits, the unit cell array can produce the data currents with 2x gray levels.